PROJECT SUMMARY/ABSTRACT Despite decades of research, preeclampsia remains a serious public health burden. The only treatment for preeclampsia is delivery, which often leads to iatrogenic preterm birth. Unfortunately, clinical symptoms do not indicate progression to severe maternal outcomes. Thus, preeclampsia remains a significant contributor to both maternal and infant morbidity and mortality. Health risks extend beyond pregnancy, as women with preeclampsia are more likely to develop cardiovascular disease later in life. There is a significant need to improve understanding of preeclampsia etiology and to define the heterogeneous nature of the syndrome. As systemic inflammation and endothelial dysfunction are hallmarks of preeclampsia, novel immune stimulating syncytiotrophoblast microvesicles (STBEVs) are implicated as potential biomarkers to monitor placental health. Small studies report elevated plasma STBEVs in preeclampsia after diagnosis and STBEVs trigger inflammation and endothelial dysfunction in experimental models. However, large scale epidemiologic investigations of STBEVs and their influence on immune activating components, such as danger associated molecular patterns (DAMPs), have not been conducted in preeclampsia prior to clinical diagnosis. Our long-term goal is to identify biomarkers that can distinguish pathophysiological preeclampsia phenotypes. The current proposal will address gaps in the literature to advance our understanding of STBEVs in preeclampsia. The central hypothesis is that circulating STBEVs lead to higher levels of circulating DAMPs and antiangiogenic molecules that trigger the clinical symptoms of preeclampsia. The current proposal will utilize a nested case-control study design and obtain plasma and prospectively collected metadata from 280 women who developed preeclampsia and 560 controls (selected by incidence density sampling) in the Screening for Obstetric and Pregnancy Endpoints cohort. The specific aims are to; 1) Determine if STBEVs are elevated in women with preeclampsia prior to clinical diagnosis. 2) Determine if maternal/fetal factors influence STBEV levels. 3) Determine the relationship between STBEV's and circulating levels of DAMPs and antiangiogenic molecules previously implicated in preeclampsia. 4) Determine if STBEVs can improve preeclampsia phenotype discrimination. This study design is an efficient approach to measure STBEVs prior to preeclampsia diagnosis. Innovative features of this study include measuring STBEVs with increased sensitivity and less sample volume than the standard developed by Knight et al in 1998, delineating the role of STBEV's in preeclampsia and utilizing latent mixture modelling to identify preeclampsia phenotypes. Our proposal is significant, as clinical symptoms do not identify women who will progress to severe outcomes and truly require induced delivery (current treatment). Thus, progression towards redefining PE may reduce unnecessary hospitalization, early delivery, and missed opportunities to prevent maternal and fetal morbidity and mortality.